Scanning transmission electron microscope and scanning transmission electron microscopy

ABSTRACT

A scanning transmission electron microscope for scanning a primary electron beam on a sample, detecting a transmitted electron from the sample by a detector, and forming an image of the transmitted electron. The scanning transmission electron microscope includes an electron-optics system which enables switching back the transmitted electron beam to the optical axis by a predetermined quantity, and a determining unit for determining the quantity based on a displacement of the transmitted electron with respect to the detector caused by the scanning of the primary electron beam.

INCORPORATION BY REFERENCE

This application is a continuation application of U.S. Ser. No.11/328,173, filed Jan. 10, 2006 now U.S. Pat. No. 7,227,144, thecontents of which are incorporated herein by reference.

The present application claims priority from Japanese application JP2005-004660 filed on Jan. 12, 2005, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning transmission electronmicroscope (: STEM). More concretely, it relates to a control device andmethod for controlling an electron beam which has passed through aspecimen.

The STEM is a device for visualizing specimen structure with asub-nanometer high space resolution. A raster scanning of an electronbeam which is converged down to a nanometer order is performed on aspecimen whose film is thinned down to a few hundreds of nm. Moreover, asignal generated from an electron-beam irradiation area is detected,then being synchronized with the raster scanning. This synchronizationallows the two-dimensional image to be formed. Examples of the signalgenerated from the electron-beam irradiation area are a transmittedelectron beam, a secondary electron beam, and characteristic X-rays.

FIG. 2 is a basic configuration diagram of the general STEM. Thedirection parallel to an optical axis 20 of the lens-barvel is definedas the Z direction, and the plane perpendicular to the optical axis isdefined as the XY plane. A primary electron beam emitted from anelectron gun 1 is accelerated up to a few hundreds of kV, then beingformed in shape by a first condenser lens 2 and a second condenser lens3. Moreover, the primary electron beam passes through a condenseraperture 4 for restricting an aperture angle of the primary electronbeam, then being focused on a specimen 21 by an upper objective lens7-1. Although an objective lens is, physically, a single lens, thespecimen 21 is set up in a gap of the objective lens. For this reason,the upper objective lens 7-1 and a lower objective lens 7-2 are assumedas the ray diagram. Here, the lens 7-1 allows focal point of the primaryelectron beam to be achieved on the specimen, and the lens 7-2 has arole of projecting the transmitted electron beam which has passedthrough the specimen 21. An irradiation lens system including theselenses permits the primary electron beam to be converged down into asub-nm diameter on the specimen 21.

The transmitted electron beam which has passed through the specimen 21is projected onto an electron-beam detection system by the lowerobjective lens 7-2 and a projection lens 9. A raster scanning of theelectron beam is performed within the XY plane by a scanning coil 5.Furthermore, a control signal for the scanning coil 5 and an outputsignal from an electron detector 14 are synchronized with each other,thereby forming a STEM image within a computer, and displaying the STEMimage on a monitor. The characteristic of the STEM is that changing thedetection signal permits various specimen information to be imaged in aneasy and convenient manner. For example, if high-angle scatteredelectrons are detected using an electron annular detector 12, ahigh-angle annular dark field (: HAADF) image can be acquired. Iflow-angle scattered electrons in proximity to the optical axis 20 areselected using an angle selection aperture 13, and if the low-anglescattered electrons selected are detected using the electron detector14, a bright field (: BF) image can be acquired.

JP-A-2001-93459 has disclosed the following technology: Namely,according to this technology, in the device configuration for acquiringan electron energy loss spectrum (: EELS) image, a change in incidentposition of the transmitted electron beam relative to the electrondetector is cancelled using a de-scanning coil. Here, this change willoccur in accompaniment with a change in incident position of the primaryelectron beam relative to the specimen.

If the incident position of the primary electron beam relative to thespecimen 21 is changed, the incident position of the transmittedelectron beam relative to the electron detector changes. Accordingly,when selecting electrons under a certain condition out of thetransmitted electrons by an aperture or a slit, and detecting theselected electrons, the transmitted electron beam displaces relative tothe electron detector. On account of this displacement, the condition ofthe transmitted electrons to be detected at the electron detectorchanges depending on the incident position of the primary electron beam.This condition change causes an artifact to occur in the STEM image. Theuse of the de-scanning coil makes it possible to suppress this artifact.In JP-A-2001-93459, however, no disclosure has been made concerning aconcrete control method for the de-scanning coil.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electronmicroscope or its application device where a higher image correction isimplemented. This implementation is accomplished by performing thede-scanning of the transmitted electron beam with an accuracy which ishigher than the one in the conventional technology.

In order to implement the high-accuracy image correction, the presentinvention actually measures the displacement quantity of the transmittedelectron beam, then controlling the de-scanning of the transmittedelectron beam on the basis of an actually-measured result. For thispurpose, in the electron microscope or an image correction systemaccording to the present invention, there are provided anactually-measuring unit for actually measuring a displacement quantityof the transmitted electron beam, a de-scanning unit for de-scanning thetransmitted electron beam, and a determination unit for determining ade-scanning quantity of the transmitted electron beam on the basis ofthe actually-measured displacement quantity. Moreover, the electronmicroscope or the image correction system feedbacks the determinedde-scanning quantity to the de-scanning unit, thereby executing theimage correction.

As the actually-measuring unit for actually measuring the displacementquantity, the electron detector, e.g., a scintillator-equipped CCDcamera, is usable. As the de-scanning unit for de-scanning thetransmitted electron beam, the use of, e.g., the de-scanning coil, ispreferable. If, however, some other appropriate unit exists, using thatunit is also preferable. The determination unit for determining thede-scanning quantity is implemented by, e.g., a method where acomputation unit for determining the de-scanning quantity on the basisof the actually-measured value is caused to execute a predeterminedalgorithm.

In a system where the de-scanning coil makes it possible to cancel thedisplacement of the transmitted electrons caused by theprimary-electron-beam scanning, the most important device performance isthe correction accuracy. In the conventional control system, however,implementation of an enhancement in the correction accuracy has requireda complicated and troublesome device adjustment. Accordingly, we havedevised the following system by digitizing the control over the scanningcoil 5: Namely, while being synchronized with a digital control signalresulting from this digitization, values in a de-scanning tableregistered in a FM (2) are outputted to the de-scanning coil 11. Here,the de-scanning table is created as follows: Positions of thetransmitted electrons before and after activating the scanning coil 5and the de-scanning coil 11 are photographed using a camera. Then, basedon a result acquired by analyzing a resultant displacement quantity ofthe transmitted electrons, the de-scanning table is created. Moreover,even if a set state of the device has been changed, it is possible todeal with this change by updating the de-scanning table. This makes itpossible to execute a high-accuracy de-scanning at any time.Furthermore, updating the de-scanning table is automatically executed inaccordance with an analysis flowchart registered in a computer 16. Thismakes the complicated and troublesome adjustment task unnecessary. If anend user has recognized that the transmitted electrons are beingoscillated by the incident-electron-beam scanning, all that the end userhas to do is to send an instruction of updating the de-scanning table.

The enhancement in the correction accuracy makes it possible to enlargethe field-of-view of the STEM image. This tremendously improves theusability of the device. Also, it becomes possible to make smaller thehole diameter of the angle selection aperture 13. This also allows anenhancement in the selection accuracy of the transmitted electron beam.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic configuration diagram of a de-scanning systemaccording to the present invention;

FIG. 2 is the basic configuration diagram of the general STEM;

FIG. 3 is a basic configuration diagram of the STEM used in a firstembodiment;

FIG. 4 is a ray diagram for illustrating the correspondence between anaxis-shift quantity Rs on the specimen and an axis-shift quantity Rd onthe angle selection aperture;

FIG. 5 is an explanatory diagram for explaining differences in theaxis-shift quantity on the angle selection aperture, the differencesbeing attributed to differences in the specimen height within theobjective lens;

FIG. 6 is a flowchart for analyzing the relationship between acontrol-value change quantity of the de-scanning coil 11 and adisplacement quantity of diffraction images;

FIG. 7 is an explanatory diagram for explaining the relationship betweenthe control-value change quantity f of the de-scanning coil 11 and thedisplacement quantity F of the diffraction images;

FIG. 8 is a flowchart for creating the de-scanning table from thedisplacement quantity of the diffraction images at the time when thecontrol value for the scanning coil 5 is changed;

FIG. 9 is a control screen for an end user;

FIG. 10 is a control screen for confirming creation steps of thede-scanning table;

FIG. 11A to FIG. 11C are de-scanning-table display screens forconfirming respective de-scanning tables created;

FIG. 12A, FIG. 12B, and FIG. 12C are an explanatory diagram forillustrating the diffraction image and the STEM image at the time whenthe de-scanning exhibits no function, an explanatory diagram therefor atthe time when the de-scanning is insufficient, and an explanatorydiagram therefor at the time when the de-scanning is being executed withan excellent accuracy, respectively;

FIG. 13 is a basic configuration diagram of the STEM used in a secondembodiment;

FIG. 14 is a basic configuration diagram of the STEM used in the secondembodiment;

FIG. 15 is a basic configuration diagram of the STEM used in the secondembodiment;

FIG. 16 is a basic configuration diagram of the STEM-EELS used in athird embodiment;

FIG. 17 is a basic configuration diagram of a de-scanning system for theSTEM-EELS used in the third embodiment; and

FIG. 18 is an explanatory diagram for illustrating processing steps foranalyzing position-shift quantity between the images.

DESCRIPTION OF THE INVENTION

In the electron microscope or the image correction system according tothe present invention, there are provided an actually-measuring unit foractually measuring a displacement quantity of the transmitted electronbeam, a de-scanning unit for de-scanning the transmitted electron beam,and a determination unit for determining a de-scanning quantity of thetransmitted electron beam on the basis of the actually-measureddisplacement quantity. Moreover, the electron microscope or the imagecorrection system feedbacks the determined de-scanning quantity to thede-scanning unit, thereby executing the image correction.

As the actually-measuring unit for actually measuring the displacementquantity, the electron detector, e.g., a scintillator-equipped CCDcamera, is usable. If the transmitted electrons enter the scintillator,photons are generated from the entrance positions, then being detectedby the CCD camera. The electron image photographed by the CCD camera issent to an image processing unit. The image processing unit analyzes thedisplacement quantity of the transmitted electron beam caused by adisplacement of the primary electron beam, then recording the analysisresult into a memory. As the de-scanning unit for de-scanning thetransmitted electron beam, the use of the de-scanning coil ispreferable. If, however, some other appropriate unit exists, using thatunit is also preferable. For example, in the STEM of a configuration asillustrated in FIG. 3, the transmitted electrons to be detected by theelectron detector 14 may be corrected by causing the position of theangle selection aperture 13 to follow the displacement quantity of thetransmitted electron beam.

Also, the transmitted electrons which are to form the STEM image may becorrected as follows: The electron detector 14 is changed to anelectron-image detector, thereby being formed into a system for formingthe STEM image by a detection signal in a specified pixel area. Then,the specified pixel area is caused to follow the displacement quantityof the transmitted electron beam, thereby correcting the transmittedelectrons. The determination unit for determining the de-scanningquantity is implemented by, e.g., the following method: Namely,measurements are made concerning the correspondence between the controlquantity of the scanning coil of the primary electron beam and thedisplacement quantity of the transmitted electron beam, and thecorrespondence between the control quantity of the de-scanning unit andthe displacement quantity of the transmitted electron beam. Then, fromthese actually-measured values, a computation unit calculates thecontrol quantity of the de-scanning unit necessary for canceling thedisplacement of the transmitted electron beam caused by the displacementof the primary electron beam. The control quantity of the de-scanningunit is stored into a record unit such as the memory together with anelectron-optics condition at the time of the measurements. At the timeof executing the de-scanning, the de-scanning control quantity which isnecessary in correspondence with an electron-optics condition at thetime of the execution is read out, then being sent to the de-scanningunit. Incidentally, if the de-scanning control quantity under theelectron-optics condition at the time of the execution has been notrecorded in the memory, a de-scanning control quantity already recordedin the memory is sent to the de-scanning unit after a necessarycomputation has been applied thereto. For example, if the magnificationdiffers between the measurement time and the execution time, a valueacquired by making the magnification correction to the de-scanningcontrol quantity already recorded in the memory is sent to thede-scanning unit.

EMBODIMENT 1

Hereinafter, the explanation will be given below concerning a firstembodiment. FIG. 3 illustrates the basic configuration diagram of theSTEM used in the present embodiment. The direction parallel to anoptical axis 20 of the housing is defined as the Z direction, and theplane perpendicular to the optical axis is defined as the XY plane. Thepresent STEM includes the following configuration components: Anelectron gun 1 for emitting a primary electron beam, a first condenserlens 2 and a second condenser lens 3 for forming in shape the primaryelectron beam emitted from the electron gun 1, a condenser aperture 4for restricting an aperture angle of the primary electron beam, ascanning coil 5 for allowing the primary electron beam to be scanned ona specimen, an objective lens 7 for allowing focal point of the primaryelectron beam to be achieved on the specimen, a specimen stage 8 forholding the specimen 21, a secondary-electron detector 6 for detectingsecondary electrons emitted from surface of the specimen 21, aprojection lens 9 for projecting a transmitted electron beam, which haspassed through the specimen 21, onto an electron-beam detection system,a detected-electron alignment coil 10 for aligning the transmittedelectrons onto the electron-beam detection system, a de-scanning coil 11for canceling a displacement of the transmitted electron beam on anangle selection aperture 13 caused by an incident-electron-beam scanningon the specimen 21, an electron annular detector 12 for detectinghigh-angle scattered electrons out of the transmitted electrons, theangle selection aperture 13 for extracting from the transmittedelectrons an electron beam having a desired detection angle, an electrondetector 14 for detecting the transmitted electrons which have passedthrough the angle selection aperture 13, an electron detection camera 15for photographing a diffraction image formed by the transmittedelectrons, control circuits for controlling the electron gun, therespective electron lenses, the respective coils, the respectiveapertures, the specimen stage, and the respective detectors, and acomputer 16 for controlling the respective control circuits. The angleselection aperture 13 and the electron detector 14 are formed into amovable-type mechanism which will deviate from the optical axis when thediffraction image is observed by the electron detection camera 15.

Next, the explanation will be given below concerning processing stepsfor acquiring the STEM image by using the device illustrated in FIG. 3.The primary electron beam is extracted from the electron gun 1 by anextraction voltage V1. Then, an acceleration voltage V0 is applied tothe primary electron beam extracted. The specimen 21 is mounted on thespecimen stage 8, then being inserted into a specimen chamber. Currentvalues for the first condenser lens 2, the second condenser lens 3, theobjective lens 7, and the projection lens 9 are set at predeterminedvalues. Although the objective lens 7 is, physically, a single lens, thespecimen is set up in a gap of the objective lens. For this reason, anupper objective lens 7-1 and a lower objective lens 7-2 are assumed asthe ray diagram. Here, the upper objective lens 7-1 allows focal pointof the primary electron beam to be achieved on the specimen, and thelower objective lens 7-2 has a role of projecting the transmittedelectron beam which has passed through the specimen 21. The primaryelectron beam is formed in shape by the first condenser lens 2 and thesecond condenser lens 3, then being focused on surface of the specimen21 by the upper objective lens 7-1. The transmitted electron beam whichhas passed through the specimen 21, after being image-formed by thelower objective lens 7-2 and the projection lens 9 one after another, isprojected onto the electron-beam detection system. Furthermore, thescanning coil 5 is controlled, thereby performing a raster scanning ofthe microscopically-converged electron beam within the XY plane on thesurface of the specimen 21. Then, scanning position information by thescanning coil 5 and an output signal from the electron detector 14 aresynchronized with each other, thereby forming a STEM image within thecomputer, and displaying the STEM image on a monitor.

The characteristic of the STEM is that changing the electrons to bedetected permits various specimen information to be imaged in an easyand convenient manner. For example, if the high-angle scatteredelectrons are detected using the electron annular detector 12, ahigh-angle annular dark field (: HAADF) image can be acquired. Iflow-angle scattered electrons in proximity to the optical axis 20 areselected using the angle selection aperture 13, and if the low-anglescattered electrons selected are detected using the electron detector14, a bright field (: BF) image can be acquired. If a diffractedelectron beam with a specific diffraction index is guided from thetransmitted electron beam into the hole of the angle selection aperture13 by using the detected-electron alignment coil 10, and if thediffracted electron beam guided is detected using the electron detector14, a dark field image with the specified diffraction index can beacquired. Also, if the secondary electrons emitted from the surface ofthe specimen 21 are detected using the secondary-electron detector 6, ahighly-accelerated secondary-electron image can also be acquired.

The secondary-electron detector 6 is configured to take in the secondaryelectrons which have been emitted in a variety of directions.Consequently, even if incident position of the primary electron beam onthe specimen 21 is changed, detection efficiency of thesecondary-electron beam seldom changes. On the other hand, thetransmitted-electron detector 14 detects the transmitted electrons insuch a manner that, depending on a diffraction direction of thetransmitted electron beam, the transmitted-electron detector 14 selectsthe transmitted electrons. For this selection to be implemented, thereis provided a projection lens system including the units such as theprojection lens 9, the detected-electron alignment coil 10, and theangle selection aperture 13. In this case, if the incident position ofthe primary electron beam on the specimen 21 is changed, orbits of thetransmitted electrons change in the projection lens system. As a result,the incident position of the transmitted electron beam on the angleselection aperture 13 changes.

FIG. 4 illustrates a change in the incident position of the transmittedelectron beam on the angle selection aperture 13 in the case where theincident position of the primary electron beam on the specimen 21 ischanged. If the incident position of the primary electron beam on thespecimen 21 is displaced by a displacement quantity Rs from on theoptical axis, the incident position of the transmitted electron beamentering the angle selection aperture 13 displaces by a displacementquantity Rd. It is possible to calculate a substantial value of thedisplacement quantity Rd from optical magnification of the lowerobjective lens 7-2 and that of the projection lens 9. Concretely, bycalculating the optical magnification Mo of the lower objective lens7-2, the optical magnification Mp of the projection lens 9, and a beamspread r on the angle selection aperture 13, the displacement quantityRd can be approximately calculated based on the following expression 1:

$\begin{matrix}{{R_{d} \cong {{R_{s} \cdot M_{o} \cdot M_{p}} + r}} = {{R_{s} \cdot \frac{L_{o}}{L_{s}} \cdot \frac{L_{p}}{L_{3} - {Lo}}} + {\alpha \cdot ( {L_{3} - L_{o}} ) \cdot \frac{L_{4} - L_{p}}{L_{p}}}}} & (1)\end{matrix}$

However, the settings of the objective lens 7 and the projection lens 9change because of a variety of factors as well. For example, FIG. 5illustrates changes in Rd which occur when z position of the specimen 21within the objective lens is changed. For simplicity, the considerationwill be given regarding the case where the projection lens is omitted.FIG. 5( a) illustrates the case where the specimen 21 is positioned atthe center of the objective lens. If the specimen 21 is positioned inthe downstream than the center of the objective lens, in order toachieve the focal point of the primary electron beam onto the specimen21, the magnetizing excitation of the objective lens 7 is loweredthereby to lengthen the focal length thereof (FIG. 5( b)). Meanwhile, ifthe specimen 21 is positioned in the upstream than the center of theobjective lens, the focal length of the objective lens 7 is shortened(FIG. 5( c)).

As described above, if the height of the specimen 21 is changed,position of the image plane of the objective lens, i.e., Lo, changes.Moreover, it is obvious that position of the image plane of theprojection lens, i.e., Lp, also changes. The expression 1 shows that, ifLo or Lp is changed, Rd will change. As factors for changing Rd, inaddition to the z-position change in the specimen, z-position change ina virtual light-source can also be mentioned. Furthermore, there aresome cases where, depending on an adjustment state of theelectron-optics system, Rd will change asymmetrically with reference tothe optical axis. For example, if an astigmatism exists in theprojection lens, the change quantity of Rd also turns out to have anastigmatism. Also, if Rd becomes larger, it becomes impossible toneglect influence of an off-axis aberration of the projection lens.Implementation of the high-accuracy de-scanning requires a system whichmakes it possible to cancel these factors in an easy and convenientmanner.

Accordingly, in order to implement the high-accuracy de-scanning, wehave devised the following system: Namely, this system actually measuresRd, then creating a de-scanning table on the basis of theactually-measured result. Moreover, based on this de-scanning table,this system controls the de-scanning coil 11. FIG. 1 illustrates asystem configuration diagram thereof. In the present system, the controlvalue for the scanning coil 5 is outputted from a DBC (: Digital BeamControl) circuit. While being synchronized with this digital controlsignal, values in the de-scanning table registered in a FM (: FrameMemory) (2) are outputted to the de-scanning coil 11. In addition to asystem for writing the output from the electron detector 14 into a FM (:Frame Memory) (1) in synchronization with the scanning coil 5, thecorrection system in the present embodiment also includes a system foroutputting the values recorded in the FM (2) in the manner of beingsynchronized with the scanning coil 5. The de-scanning table is causedto have the same pixel number as that of the STEM image. It is moreefficient to make the pixel number of the de-scanning table and that ofthe STEM image equal to each other. This is because no calculation isneeded when reading the control values from the de-scanning table. If,however, an algorithm is added which is used for performing areading-through or overlapping of the control values, the pixel numberof the de-scanning table and that of the STEM image may differ from eachother.

At the time of the factory shipping, it is preferable that thede-scanning table measured by electron-optics simulation values or thesame kind of device be stored into the frame memory. By using thisde-scanning table, a beginner who has not mastered a proofreading methodfor the de-scanning table executes the de-scanning. It is advisablethat, after mastering the proofreading method, the beginner create thede-scanning table by using the most up-to-date measurement valuesthereby to enhance the de-scanning accuracy. Incidentally, if a deviceinstaller performs the proofreading of the de-scanning table at the timeof the installment, it is allowable that the frame memory at the time ofthe factory shipping be left blank.

The de-scanning table is created as follows: Diffraction images beforeand after the activations of the scanning coil and the de-scanning coilare photographed using a camera. Then, based on a result acquired byanalyzing a resultant displacement quantity of the diffraction images bythe image processing, the de-scanning table is created. Various patternmatching methods, such as phase-limited correlation method,normalization correlation method, and least-squares method, are usableas the analysis method for analyzing the displacement quantity betweenthe diffraction images. Incidentally, the phase-limited correlationmethod has been used for the displacement-quantity analysis at thistime. Hereinafter, referring to FIG. 18, the explanation will be givenbelow concerning this phase-limited correlation method. Assume twopieces of discrete images S1(m, n) and S2(m, n) with a position shiftD=(D_(x), D_(y)) existing therebetween, and describe S1(m, n) as S1(m,n)=S2(m+D_(x), n+D_(y)). Let two-dimensional discrete Fouriertransformations of S1(m, n) and S2(m, n) be S1′(k, l) and S2′(k, l),respectively. Since a formula F{S(m+D_(x), n+D_(y))}=F{S(m, n)}exp(iD_(x)•k+iD_(y)•l) exists in the Fourier transformation, S1′(k, l)can be converted into S1′(k, l)=S2′(k, l) exp(iD_(x),•k+iD_(Y)•l).

Namely, the position shift between S1′(k, l) and S2′(k, l) can beexpressed by the phase difference exp(iD_(x),•k+iD_(y)•l)=P′(k, l).P′(k, l) is also a wave whose period is equal to (D_(x), D_(y)). As aresult, a δ-function-like peak occurs at the position of (D_(x), D_(y))in an analysis image P(m, n) which results from applying an inverseFourier transformation to the phase-difference image P′(k, l).Incidentally, instead of eliminating all the information on amplitude, alog or √{square root over ( )} processing is applied to the amplitudecomponent of S1′(k, l)·S2′(k, l)*=|S1′||S2′|exp (iD_(x)•k+iD_(y)•l),thereby calculating an image whose amplitude component is suppressed.Then, applying the inverse Fourier transformation to this image alsocauses a δ-function-like peak to occur at the position of (D_(x), D_(y))of the position-shift vector. Accordingly, the position-shift analysismay also be performed using this image. Moreover, applying the Fouriertransformation to the phase-difference image P′(k, l) also causes aδ-function-like peak to occur at (−D_(x), −D_(y)). Consequently, theposition-shift analysis may also be executed using the Fouriertransformation image of the phase-difference image P′(k, l).

It can be assumed that only the δ-function-like peak exists in theanalysis image P(m, n). This condition allows the position of theδ-function-like peak to be determined with an accuracy of digits to theright of the decimal point by using a center-of-gravity positioncalculation or function fitting. Also, the portion other than theδ-function-like peak can be regarded as a noise. Accordingly, theproportion of intensity of the δ-function-like peak relative tointensity of the entire analysis image P(m, n) can be regarded as thecoincidence degree between the images. In the conventionalposition-shift analysis methods, it was difficult to evaluatereliability of the position-shift analysis result. Also, there was ashortage of the frequency component needed for the analysis.Accordingly, if a wrong position-shift quantity is outputted, it turnsout that the analysis flowchart will proceed based on this wrongposition-shift quantity. In contrast thereto, in the presentposition-shift analysis method, the coincidence degree is outputted.Consequently, it is possible to execute the following analysisflowchart: Namely, a lower-limit value of the coincidence degree is set.Then, if the coincidence degree is found to be smaller than thelower-limit value, the coincidence degree is eliminated as a value whichcould not be analyzed. Moreover, the coincidence degree is automaticallyinterpolated by an analysis result in proximity thereto.

FIG. 6 illustrates a flowchart for determining a control-value changequantity of the de-scanning coil 11 needed for displacing a diffractionimage by a predetermined quantity. Since the de-scanning coil 11includes an x deflection and a y deflection, the control value for thede-scanning coil 11 is specified as f=(fx, fy). First, a referencediffraction image is photographed at f0=(0, 0). After that, diffractionimages at several control values fi=(fxi, fyi) are photographed, thenanalyzing displacement quantities Fi=(FXi, FYi) relative to thereference diffraction image by using the image processing. A measurementcondition therefor is set. For example, if movable range of thede-scanning coil is set as being ±1024 digits in the fx deflection and±1024 digits in the fy deflection, a 121-point measurement condition isset in a lattice-like manner such that 11 points from −1000 digits to+1000 digits with a 100-digit spacing are specified to fx, and 11 pointsfrom −1000 digits to +1000 digits with a 100-digit spacing are specifiedto fy. Next, a conversion expression is acquired which determines thecontrol-value change quantities of the de-scanning coil 11 needed fordisplacing the diffraction image by the predetermined quantities frommeasurement values acquired. No electromagnetic-field lens is providedbetween the de-scanning coil and the camera. Accordingly, it has beenassumed that the conversion expression between the control-value changequantities of the de-scanning coil 11 and the displacement quantities ofthe diffraction image is describable by a 2×2 matrix A. Concretely,assuming the direction tx and pitch a of the fx deflection by thede-scanning coil, and the direction ty and pitch b of the fy deflectionthereby (refer to FIG. 7), the following expression 2 is assumed:

$\begin{matrix}{\begin{bmatrix}{FX}_{i} \\{FY}_{i}\end{bmatrix} = {\begin{bmatrix}{a \cdot {\cos( t_{x} )}} & {b \cdot {\cos( t_{y} )}} \\{a \cdot {\sin( t_{x} )}} & {b \cdot {\sin( t_{y} )}}\end{bmatrix} \cdot \begin{bmatrix}{fx}_{i} \\{fy}_{i}\end{bmatrix}}} & (2)\end{matrix}$

Based on the measurement result, the parameters of the matrix A areoptimized so that ||Fi-A•fi|| becomes equal to its minimum. An inversematrix A⁻¹ of the optimized matrix A is calculated. The control-valuechange quantities fi of the de-scanning coil 11 needed for displacingthe diffraction image by the displacement quantities Fi are calculatedby multiplying the displacement quantities Fi of the diffraction imageby A⁻¹. Incidentally, if a shift is large which exists between thedisplacement quantities calculated from the control-value changequantities of the de-scanning coil 11 by using the expression 2 and theactually-measured displacement quantities, the case is conceivablewhere, e.g., the axis deviation is large, and where the activation ofthe electrons changes in a vortex-like manner. In that case, it isnecessary to add a term of the off-axis aberration to the conversionexpression.

Next, the de-scanning table is created from a displacement quantity ofthe diffraction image caused by the control-value change quantity of thescanning coil 5. Since the objective lens 7 and the projection lens 9exist between the scanning coil 5 and the camera 15, there exists apossibility that the displacement quantity of the diffraction image maycomplicatedly change depending on setting conditions therefor.Accordingly, instead of trying to fit, into a mathematical expression,the displacement quantity of the diffraction image caused by thecontrol-value change quantity of the scanning coil 5, it has beendecided that the measurement values be used just as they are.Incidentally, it is insufficient to measure the same pixel number asthat of the STEM image. Also, it can be assumed that the electromagneticfield will never change so steeply. Consequently, it has been decidedthat measurement points be provided with a proper spacing settherebetween, and that interpolation be performed using the splineinterpolation. FIG. 8 illustrates a flowchart therefor. Since thescanning coil 5 also includes an x deflection and a y deflection, thecontrol value for the scanning coil 5 is specified as s=(sx, sy). Here,it is assumed that movable range of the scanning coil 5 has been set asbeing 1 digit to 640 digits in the sx direction and 1 digit to 480digits in the sy direction. A reference diffraction image at the centerof the field-of-view, i.e., at s=(320, 240) digits, is photographed.After that, diffraction images at several control values si =(sxi, syi)for the scanning coil 5 are photographed, then analyzing displacementquantities Si =(SXi, SYi) relative to the reference diffraction image byusing the image processing.

As the measurement points, for example, 192 points are specified in alattice-like manner such that 16 points from 20 digits to 620 digitswith a 40-digit spacing are specified in the sx direction, and 12 pointsfrom 20 digits to 460 digits with a 40-digit spacing are specified inthe sy direction. This specification makes it possible to measure, atthe respective points, the displacement quantities of the diffractionimage caused by the control-value change quantities of the scanning coil5. Consequently, the control-value change quantities of the de-scanningcoil 11 needed for canceling the above-described displacement quantitiesof the diffraction image are determined from the expression 2.Furthermore, the control-value change quantities determined areallocated onto a table whose pixel number is equal to the pixel number(i.e., 640×480) of the STEM image in accordance with the control valuesof the scanning coil 5 at the respective points. The value at a positionto which the actually-measured result has been not allocated isinterpolated by a method such as the spline interpolation, therebycompleting the de-scanning table.

The de-scanning table simultaneously stores therein the STEMmagnification with which the de-scanning table had been created. Thesetting of the scanning range of the primary electron beam, i.e., theSTEM magnification, is modified many times during the specimenobservation. It is quite inefficient to create the de-scanning tableevery time the setting is modified. Accordingly, the values in thede-scanning table are multiplied by a value which is inverselyproportional to the magnification, then being outputted to thede-scanning coil. The magnification at the time when the de-scanningtable had been created is recorded. When executing the de-scanning, themagnification at present is read. Moreover, the values in thede-scanning table are multiplied by the inverse of ratio of the readmagnification relative to the recorded magnification, then beingoutputted to the de-scanning coil. If the oscillation of the diffractionimage due to the primary-electron-beam scanning becomes conspicuousbecause the difference between the read magnification and the recordedmagnification is large, re-creating the de-scanning table will becarried out.

The above-described creation of the de-scanning table is automaticallyexecuted in accordance with the flowchart in FIG. 8. This makes itunnecessary for a user to perform the complicated and troublesome task.All that the user has to do is to issue the instruction by using ade-scanning table update icon. FIG. 9 illustrates a configurationexample of the main control screen. In the system configuration in FIG.3, the main control screen in FIG. 9 is displayed on the monitor of thePC 16. The main control screen illustrated in FIG. 9 displays thereonrequirements such as the STEM image, the diffraction image, and the iconfor issuing the instruction of creating the de-scanning table. Thejudgment on execution of the de-scanning table update is made mainlybased on the diffraction image. As illustrated in FIG. 12A, if thediffraction image is being tremendously oscillated by theprimary-electron-beam scanning, and if the transmitted electron beam tobe defected deviates from the angle selection aperture 13, updating thede-scanning table can be said to be absolutely necessary. As illustratedin FIG. 12B, if the diffraction image is being slightly oscillated, ajudgment which meets an observation purpose should be made.Incidentally, FIG. 12C illustrates the diffraction image and the STEMimage at the time when the de-scanning has been executed with anexcellent accuracy.

For example, when measuring the substantial configuration of a specimen,there exists no necessity for updating the de-scanning table. In thecrystalline structure analysis, however, updating the de-scanning tableis more advisable even if a slight oscillation were to be observed inthe diffraction image. Clicking on or double-clicking on the de-scanningtable update icon allows the most up-to-date de-scanning table to becreated in accordance with the flowchart in FIG. 8. After that, thede-scanning coil will be controlled in accordance with this de-scanningtable. Even the present device allows the most up-to-date de-scanningtable to be created in a few minutes. Decreasing the number of themeasurement points allows the most up-to-date de-scanning table to becreated in a few tens of seconds.

Incidentally, all that the end user has to do is to issue theinstruction by using the de-scanning table update icon. Depending on theuser, however, it is necessary to confirm whether or not the de-scanningtable has been created correctly. Several sub screens therefor areprepared. FIG. 10 illustrates an example of the sub control screen. Inaddition to the de-scanning table update icon for issuing theinstruction of creating the de-scanning table in accordance with theflowchart in FIG. 8, the following two icons are added: Aconversion-expression update and adjustment icon for issuing aninstruction of updating the conversion expression in accordance with theflowchart in FIG. 6, and a de-scanning execution icon for instructingon/off of the de-scanning operation based on the de-scanning table.

Also, there is provided a display unit for digital-displaying thecontrol values for the respective coils and the displacement quantitiesanalyzed by using the image processing. As the display function, it isalso advisable that the control values for the scanning coil 5 beindicated by circle marks on the STEM image photographed immediatelybefore the control. A position whose analysis has been already finishedis indicated by a hollow circle mark, and a position whose analysis isunder way at present is indicated by a circle mark with another displaycolor. The analysis results of the displacement quantities of thediffraction image are indicated by cross-character marks on thediffraction image. The control values for the de-scanning coil 11 arealso indicated by circle marks on a two-dimensional map. This makes iteasier to intuitively understand progression state of the analysis andcorrespondence between the control values for the de-scanning coil andthe analysis results of the displacement quantities. There is alsoprovided a set unit for specifying the measurement points.

Also, there is provided a display unit for displaying the createdde-scanning table (FIG. 11A to FIG. 11C). The analysis results of thedisplacement quantities of the diffraction image are displayed astwo-dimensional images. This display makes it easier to performconfirmation of the analysis results. The analysis results (FX, FY) ofthe diffraction-image displacement quantities at the coil control values(fx, fy) are respectively displayed as the two-dimensional images. Thechanges in the displacement quantities can be assumed to be smooth. As aresult, if the displacement quantities are analyzed correctly, smoothanalysis results are displayed as illustrated in FIG. 11A. If thedisplacement quantities could not be analyzed because the coilcontrol-value change quantities are too large and thus the displacementquantities of the diffraction image become too large, as illustrated inFIG. 11B, the analysis results steeply change in proximity to theanalysis-incapable locations.

When performing the interpolation processing in order to create thede-scanning table from the displacement quantities, the interpolationprocessing needs to be performed in such a manner that the result at apoint which could not be analyzed is excluded, and that the result at apoint which could be analyzed is utilized. The judgment as to whether ornot the displacement quantities could be analyzed can be made from thecorrelation values between the images. The confirmation as to whether ornot the de-scanning control values on the peripheral portions arecorrectly evaluated by this interpolation processing is also performedon the above-described two-dimensional image display of the de-scanningtable. If, as illustrated in FIG. 11C, the values on the peripheralportions are not correctly evaluated, it is required to modify themeasurement condition and to perform the analysis once again.

As having been explained so far, in the system where the de-scanningcoil makes it possible to cancel the displacement of the diffractionimage caused by the primary-electron-beam scanning, the most importantdevice performance is the correction accuracy. In the conventionalcontrol system, however, implementation of an enhancement in thecorrection accuracy has required a complicated and troublesome deviceadjustment. Accordingly, we have devised the following system: Namely,the de-scanning table is created from the actually-measured results ofthe displacement quantities of the diffraction image. Then, thede-scanning coil is controlled in a manner of being synchronized withthe primary-electron-beam scanning. This system has allowed simultaneousimplementation of both the enhancement in the correction accuracy andthe simplicity of the device adjustment. The enhancement in thecorrection accuracy makes it possible to enlarge the field-of-view ofthe STEM image. Also, it becomes possible to make smaller the holediameter of the angle selection aperture. This also allows anenhancement in the selection accuracy of the transmitted electron beam.

EMBODIMENT 2

In the first embodiment, as the de-scanning coil 11, the one-stagedeflection coil has been used which is set up between the projectionlens 9 and the electron detector 14. It is also possible, however, tocarry out the de-scanning by using some other coil. For example, asillustrated in FIG. 13, carrying out the de-scanning by using atwo-stage deflection coil is also allowable. The two-stage deflectionexhibits a disadvantage that the deflection quantity becomes smallerthan that of the one-stage deflection. However, the two-stage deflectionexhibits an advantage that it becomes possible to adjust both of theangle and the position of the electron beam.

Also, in order to clarify the functions of the respective coils, thedetected-electron alignment coil 10 and the de-scanning coil 11 havebeen described in the separate manner. It is also possible, however, toimplement these functions by using a single coil. Namely, if it iswished to simplify the coil control mechanism, integrating the coilsinto the single coil is advisable. Also, it is also possible to set theposition of the de-scanning coil between the objective lens and theprojection lens. This setting, however, complicates the correspondencebetween the control-value change quantities of the de-scanning coil 11and the displacement quantities of the diffraction image.

EMBODIMENT 3

FIG. 16 illustrates a basic configuration diagram of the STEM-EELS usedin the present embodiment. The EELS is installed on the lower portion ofthe STEM. In order to allow the transmitted electron beam to be detectedby the EELS, as is the case with the angle selection aperture 13 and theelectron detector 14, the electron detection camera 15 is also formedinto a movable-type mechanism. The EELS includes a quadrupole lens 31,an accelerator 32, an electron spectrometer 33, a quadrupole magnifyinglens 34, an energy slit 35, a mapping-use electron detector 36, aspectrum-use electron detector 37, and control units therefor.

Next, the explanation will be given below concerning processing stepsfor acquiring an EELS image by using the device illustrated in FIG. 16.First of all, the STEM image is acquired in accordance with theprocessing steps illustrated in the first embodiment. Moreover, atransmitted electron beam to which an energy spectroscopy is to beapplied is selected using the angle selection aperture 13. Then, theelectron detector 14 and the electron detection camera 15 are deviatedfrom the optical axis, thereby allowing the transmitted electron beam toenter the EELS. The electrons which have entered the EELS areenergy-dispersed by the electron spectrometer 33, then being magnifiedby the quadrupole magnifying lens 34, and being projected onto thedetectors. Furthermore, at first, the mapping-use electron detector 36and the energy slit 35 are deviated from the optical axis, thusacquiring the energy spectrum by using the spectrum-use electrondetector 37. Then, the energy spectrum is displaced in parallel by usingthe accelerator 32, thereby making an adjustment so that the transmittedelectron beam having an energy width to be detected will pass throughthe energy slit 35. After the adjustment, the energy slit 35 and themapping-use electron detector 36 are inserted onto the optical axis.This insertion causes an output signal from the mapping-use electrondetector 36 and the primary-electron-beam scanning signal to besynchronized with each other, thereby making it possible to acquire theEELS image.

If the position of a diffraction image on the EELS incident surface ischanged by the primary-electron-beam scanning, orbits of the transmittedelectrons which are passing through inside the EELS change. As a result,positions of the transmitted electrons projected onto the electrondetectors change. On account of this, a transmitted electron beam whoseenergy width differs from a predetermined energy width will pass throughthe energy slit 35, eventually entering the mapping-use electrondetector 36. In order to prevent this phenomenon from occurring, thereis provided a de-scanning coil 11 for canceling the displacement of thediffraction image on the EELS incident surface, the displacement beingcaused by a change in the incident position of the primary electron beamon the specimen. Control over this de-scanning coil 11 is performedusing the de-scanning coil control system illustrated in FIG. 1. Acontrol value for the scanning coil 5 is outputted from the DBC circuit.While being synchronized with this digital control signal, values in ade-scanning table registered in the FM (2) are outputted to thede-scanning coil 11. Here, the de-scanning table is created as follows:Diffraction images before and after activating the scanning coil 5 andthe de-scanning coil 11 are photographed using a camera. Then, based ona result acquired by analyzing a displacement quantity of thediffraction images, the de-scanning table is created. First, adisplacement quantity of the diffraction images caused by thede-scanning coil 11 is analyzed. From this result, a conversionexpression is acquired which is used for determining a control-valuechange quantity of the de-scanning coil 11 needed for displacing thediffraction image by a predetermined quantity. Next, a displacementquantity of the diffraction images caused by the scanning coil 5 isanalyzed. From the above-described conversion expression, control-valuechange quantities of the de-scanning coil 11 needed for cancelingdisplacement quantities of the diffraction image are determined, thenbeing recorded into the FM (2) as the de-scanning table. The controlvalues read from the de-scanning table are sent to the de-scanning coil11 in response to the incident position Rs of the primary electron beamon the specimen. This makes it possible to execute the high-accuracyde-scanning.

EMBODIMENT 4

In the third embodiment, the explanation has been given concerning thetechnique where the use of the FM (2) and the de-scanning coil 11 makesit possible to correct the position change in the diffraction image onthe EELS incident surface caused by the primary-electron-beam scanning.In the present embodiment, the explanation will be given below regardinga technique where the use of the accelerator 32 makes it possible tocorrect the position of the transmitted electron beam relative to theenergy slit 35. FIG. 17 illustrates a configuration diagram of a controlsystem for the de-scanning coil 11 and the accelerator 32. A de-scanningtable to be used for the de-scanning coil 11 is created in accordancewith the processing steps described in the first embodiment, then beingrecorded into the FM (2). Moreover, while being synchronized with thescanning coil 5, values in the de-scanning table registered in the FM(2) are sent to the de-scanning coil 11. In the EELS, the image on theincident surface reaches the emission surface in a manner of beingmagnified. As a result, the incident position of or the angle change inthe transmitted electron beam on the incident surface is projected ontothe energy slit 35 in a manner of being magnified inside the EELS. Inorder to correct this phenomenon so that the transmitted electron beamhaving a predetermine energy width will pass through the energy slit,the position of the transmitted electron beam relative to the energyslit 35 is corrected using the accelerator 32.

In order to implement this correction by using the accelerator, a FM(2)′ is provided. Then, taking advantage of zero-loss spectrum, adisplacement of the energy spectrum caused by the primary-electron-beamscanning is measured. Next, control values of the accelerator 32 neededfor canceling this displacement are determined, then being stored intothe FM (2)′. The data stored into the FM (2)′ are the data which aremeasured while correcting the position of the diffraction image on theEELS incident surface by using the FM (2) and the de-scanning coil 11.It is required to update the data in the FM (2)′ without fail wheneverthe data in the FM (2) are modified. Furthermore, while beingsynchronized with the control over the scanning coil 5, the data in theFM (2) are sent to the de-scanning coil 11. Simultaneously therewith,the data in the FM (2)′ are sent to the accelerator 32. This makes itpossible to enhance the de-scanning accuracy even further.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning transmission electron microscope for scanning a primaryelectron beam on a sample, detecting a transmitted electron from thesample by a detector, and forming an image of the transmitted electron,comprising: an electron-optics system which enables switching back thetransmitted electron beam to the optical axis by a predeterminedquantity; and a determining unit for determining the quantity based on adisplacement of the transmitted electron with respect to the detectorcaused by the scanning of the primary electron beam.
 2. A scanningtransmission electron microscope according to claim 1, furthercomprising: a measuring unit for measuring a displacement quantity ofthe transmitted electron by the scan of the primary electron beam.
 3. Ascanning transmission electron microscope according to claim 1, whereinthe determining unit comprises a de-scanning table in which a quantityof the displacement and a control value for controlling a quantity ofswitching back of the primary electron.
 4. A scanning transmissionelectron microscope according to claim 1, wherein the electron-opticssystem further comprises a de-scanning coil for switching back theprimary electron beam.
 5. A scanning transmission electron microscopeaccording to claim 3, further comprising: a monitor for displaying theimage of the transmission electron.
 6. A scanning transmission electronmicroscope according to claim 5, wherein an icon for instructing anexecution of the switching back operation of the transmitted electron isdisplayed on the monitor.
 7. A scanning transmission electron microscopeaccording to claim 5, wherein a content of the de-scanning table isdisplayed on the monitor.
 8. A scanning transmission electron microscopeaccording to claim 2, wherein the measurement unit includes a camera forphotographing a diffraction image of the transmitted electrons, and animage processing unit for analyzing a displacement quantity between thediffraction images obtained by the camera.